1. Field of the Invention
The present invention relates to method and apparatus for performing perforating operations, and, more particularly, to performing such operations in a material which is naturally fractured or which has a low tensile strength.
2. Description of the Prior Art
For purposes of enhancing production from a subterranean formation, a perforating gun typically is lowered down into a wellbore that extends through the formation. A perforating gun comprises a plurality of radially-oriented shaped charges which are detonated to form perforations in the formation proximate the wellbore. The shaped charges typically are placed at points along a helical spiral that extends around a longitudinal axis of the perforating gun.
It is known that charge penetration into the hydrocarbon-bearing formation is a major determinant of well productivity. Extensive investigations have previously been conducted to characterize penetration, mainly into sandstone formations. Recent charge penetration experiments into coal have revealed surprising results that may be associated with the complex cleat or natural fracture system apparent in many kinds of coal. It is believed that these fractures in coal may adversely affect charge penetration performance. Specifically, such detriment may be due to shock passage through these fractures ahead of the jet. Such effects are expected to primarily occur when penetration velocity is subsonic with respect to the prevailing acoustic velocity of coal; i.e., during later stages of penetration when incoming jet velocity is lowest.
Classical hydrodynamic theory has since the 1940's been applied to the analysis shaped charge penetration. When a jet (of density ρj), traveling at velocity V penetrates a target having a density ρt, the jet-target interface will advance at a penetration velocity U. Penetration velocity is always some fraction of the incoming jet velocity; specifically:U/(V−U)=sqrt(ρj/ρt)The magnitude of U, relative to the prevailing local acoustic velocity (C0) of the target material, determines whether the penetration is sub- or super-sonic. If U<C0, the penetration is said to be subsonic, and the shock wave formed by the penetration event will separate from the interface and advance ahead into the target. This separated wave can alter the state of the target into which subsequent jet portions enter.
Even for jet penetration which is slightly supersonic, the shock wave may detach due to shock velocity exceeding the acoustic velocity. Furthermore, an attached shock will tend to separate from the incoming jet, if the jet itself is decelerating (as is the case with real shaped charge jets).